How to know that a partition is ready when using Structured Streaming

How to know that a partition is ready when using Structured Streaming

When using structured streaming, we use "partitionBy" api to partition the
output data, and use the watermark based on event-time to handle delay
records, but how to tell downstream users that a partition is ready? For
example, when to write an empty "hadoop.done" file in a paritition
directory?